1. Field of the Invention
This invention relates to a method for producing a dense sintered silicon carbide body from polycarbosilane, and more particularly, to a method of producing a dense sintered silicon carbide body having density of 2.4 to 3.2 g/cm.sup.2 and flexural strength of 13 to 98 kg/mm.sup.2 from polycarbosilane which is insoluble in solvents and unmeltable owing to its melting or softening temperature being higher than its thermal decomposition temperature.
This invention includes also a method capable of reducing the sintering temperature by up to 100.degree. C. by specifying the addition time of sintering aids.
2. Prior Art
Taking the place of conventional oxide sintered ceramics which have been hitherto obtained by sintering metallic oxides such as Al.sub.2 O.sub.3, SiO.sub.2, MgO, ZrO.sub.2, TiO.sub.2 and the like, separately or in their mixed powder, a new ceramic technology capable of obtaining sintered bodies of carbide such as SiC is now evolving. As for the powdered SiC, it has become possible to obtain an elaborate self-sintering body of SiC by a hot press method in which the powdered SiC is used together with a few percent of bonding agent such as alumina, boron, metallic silicon, tungsten carbide, and the like. It seems that such types of sintered bodies are high in compressive strength, and good in thermal shock resistance as well as in oxidation resistance.
In this prior art, however, the application of bonding agents leaves impurities other than SiC in the products, for example, aluminum oxide, silicon, boron, free carbon, tungsten carbide, silicon nitride, and so on, so that the thus sintered body is lower in strength at an elevated temperature and had to be laid under restriction in use.
To get over such inconveniences, a method was filed with the Japanese Patent Application No. 115965/1975, wherein powdered SiC was mixed with polycarbosilane prepared by polymerizing one or more of dimethyldichlorosilane, dodecacyclohexasilane, 1,1,3,3-tetramethyl-1,3-disilacyclobutane and so on and then fired in a nonoxidizing atmosphere to convert the polycarbosilane into SiC. This polycarbosilane employed as a binder in this method differs from that employed as a starting material in out invention only in the former's solubility and fusibility. We will refer to this method hereinafter as the invention "A". There is proposed another technique with the Japanese Patent Application No. 134122/1975 (hereinafter referred to as the invention "B"). This has for its object to increase the strength the sintered body of SiC obtained with one step according to the invention A by refiring it after having impregnated the above-mentioned bonding agents at least one time thereinto.
In another Japanese Patent Application No. 77567/1975 (hereinafter referred to as the invention "C"), there was proposed a method whereby the initial product of SiC procured by heating polycarbosilane in a nonoxidizing atmosphere was ground into a powdered body in which a high molecular compound of organosilicon as a bonding agent was mixed, and, after molding this mixture, was fired in a nonoxidizing atmosphere. Further, in another pending patent application (hereinafter referred to as the invention "D"), which the applicant of the present application had filed in the United States prior to this application, the applicants of the present application contrived a method wherein the insoluble and unmeltable polycarbosilane powder selected for a starting material is placed in a hot press mold, thereby promoting the thermal decomposition of the polycarbosilane by controlling temperature and pressure, thus creating silicon carbide and then sintering it.
In the above-mentioned inventions from A to D, however, the following shortcomings exist